A spray dryer is the device which is used for the drying of all types of materials and mostly the thermolabile, hygroscopic drugs or the materials that under goes chemical decomposition.
In the spray dryer the liquid to be dried is sprayed in the form of mist. The minute droplets are readily evaporated and gets converted in to the solid particles, which fall to the bottom of the chamber.
The vapours are transferred in to the separator where the fine dry particles which are carried along with the vapours are separated and collected. Spray dryers are available in many forms and deigns.
A typical spray dryer consist of a drying chamber which is just like the cyclone separator, so as to ensure the good circulation of a air to facilaite heat and mass transfer and also to ensure that the dried particles are separated by the centrifugal action.
The character of the particles depends on the liquid to be converted in the form of droplets. As such it is important to use the right type of the atomiser.
Two types of atomiser are used, they are
- Jet atomiser
- Rotary atomiser
Jet atomiser are easily blocked resulting in variation of the droplets size. Rotary atomiser are preferred to avoid this problems.
- In the spray dryer the fluid to be driedis atomised in to the fine droplets, which are thrown radially in to a moving stream of hot gas.
- The temperature of the droplets is immediately increased and fine droplets get dried instantaneously in the form of spherical particles.
- This proces complets in a few seconds before the droplets reach the wall of the dryer.
The construction of the spray dyer consists of a large cylindrical drying chamber with a short conical bottom made up of stain less steel with the diameter of the drying chamber ranges between 2.5 to 9 m and height is 25 m or more. An inlet for the hot air is palced in the roof of the chamber. Another inlet spray disk atomiser is set in the roof. The spray disk atomiser is about 300 mm in diameter and rotates at a speed of 3000 to 50000 revolutions per min. Bottom of the dryer is connected to a cyclone separator.
- Atomization of the liquid
- Drying of the liquid droplets
- Recovery of the dried products
Atomization of the liquid to form liquid droplets:
- The feed is introduced through the atomizer either by gravity or by using suitable pump to form fine droplets.
- The properties of the final products depend on the droplets form, hence the selection of the type of a atomizer is important.
- Atomizer of any type, pneumatic atomizer, pressure nozzle and spinning disc atomizer may be used.
- The rate of feed is adjusted in such a way that the droplets should be completely dried reaching the walls of the drying camber.
- At the same time, the product should not be over heated.
Drying of the liquid droplets:
- Fine droplets are dried in the drying camber by supplying hot air through the inlet.
- The surface of the liquid drop is dried immediately to form a tough shell.
- Further the liquid inside must escape by diffusing through the shell at a particular rate.
- At the same time heat transfer from outside to inside takes place at a rate greater than liquid diffusion rate.
- As a result heat inside mounts up which allows the liquid to evaporates at faster rate.
- This tendency of a liquid leadsto rise in the internal pressure which causes the droplets to swell.
- The shells thickness decreases where as permeability for vapour increases.
- If the shell is neither elastic nor permeable it ruputur and the internal pressure escapes.
- The temperature of the air is adjusted in such a way that the droplets should be compeletly dried before reaching the walls of the drying chamber.
- The products should not be over heated at the same time.
Recovery of the dried products:
- Centrifugal force of the atomizer drives the droplets to follow helical path.
- Particles are dried during their journey and finally fall at the conical bottom.
- All these processe are completed in a few seconds.
- Particles size of the final products ranges from the 2 to 500 mm.
- Particles size depends on solid content in the feed, liquid viscosity, feed rate and disc speed.
- Spray dryer of maximum size have got evaporating capacity up to 2000 kg per hour.
- Spray dryer can be used for drying of ny substances both in solution or in suspension.
- Spray dryer are very useful for the drying of thermolabile substances.
- Citric acid, borax, sodium phosphate, hexamine, gelatine and extracts are dried by a spray dryer.
- The suspensions of starch, barium sulphate and calcium phosphate are also dried by the spray dryer.
- Milk, soap and detergents too are dried by a spray dryer.
- Spray dryer are used compulsorily if:
- The products is a better form than that obtained by any other dryer.
- The quantity of the materials to be dried is large.
- The products is hygroscopic or undergoes chemical decompisition.
- Some of the products that are dried using the spray dryer are acacia, adrenaline, bacitracin, blood, boric acid, calcium sulphate, coffee extract, dextran, fruit juices, ferrous sulphate, pepsin, pancreatin, plasma, serum, soaps, sodium phosphate, starch, sulphur, vaccines vitamines, yeast etc.
- In this the drying is a continuous proces and drying is very rapid. Drying completes within 3 to 30 seconds.
- Labour costs are low as it combines the function of an evapotator, crystaillzer, a dryer, a size reduction unit and a classifier.
- By using suitable atomiser the products of uniform and controlled size can be obtained. Free flowing products of uniform spheres is formed which is very convenient for tabletting process.
- Fine droplets formed provides larges surface area for heat and mass transfer. Product shows excellent solubility.
- Either the solutions or suspensions ar thin paste can be dried in one step to get the final product ready for package.
- It is suitable for the drying of the sterile products.
- Reconstituted products appears more or less similar to the fresh materials.
- Globules of an emlusion cn be dried with the dispersed phase inside and layer of the continuous phase outside. On the reconstitution the emulsion will be formed.
- The spray dryers is very bulky (height of 25 m and diameter of 9 m) and expensive.
- Such a huge equipment is not always easy to operates.
- The thermal efficiency is low, as much heat is lost in the discharged gases.
- This dryer can be constructed in such a way as to suit sterile products.
- It can be operated under closed conditions to recover solvents.
- It can be operated under oxygen free environment.
- It can be constructed in such a way that the flow of liquid and gas be co current, counter current or a combination of both in the same unit.
- The same equipment can be used for spray congealing.
- Mehta RM, Pharmaceutics-I: introduction to the drying process (1996), Page no: 194.
- C.v.s subrahmanyam et al., pharmaceutical engineering principles and practices-filtration, 2001, page no. 394 - 397.